1. Field of the Invention
This invention relates to oscillator circuitry and more specifically to oscillators structured to compensate for supply voltage and transistor parameter variations.
2. Description of the Prior Art
In the past oscillator circuits have been used for many purposes. The present invention is intended for use with microcomputer and microprocessor circuits to produce the clock signals that govern the execution of operations between the microprocessor and microcomputer. The oscillators designed for microcomputers/microprocessors in the past have been implemented with metal oxide silicon field effect transistors (MOSFETS) on the chip themselves. Certain oscillators have required external excitation while others do not have such a requirement. The present invention does require an external resistor and capacitor connection.
In previous microcomputer/microprocessor oscillator circuits, a depletion transistor has been connected to a voltage supply and an enhancement transistor is connected to this depletion transistor to pull the depletion device down to generate an oscillating output pulse. When this enhancement transistor is turned on there is a large current through the depletion device. When the oscillator is to provide a high frequency, this depletion device must be quite large. In a microcomputer/microprocessor circuit this oscillator circuit would require a disproportionately large amount of power and area.
The microcomputer/microprocessor oscillators in the prior art also have problems with frequency stability in the presence of varying transistor characteristics, supply voltage and temperatures. The external devices connected to the oscillator circuit to control the frequency of the oscillator circuit may be adjusted for each individual circuit to compensate for variations in the transistor characteristics and even the power supply voltage. However these adjustments do not compensate for variations in temperature. Since this oscillator frequency directly controls the rate of execution of operations in the microcomputer/microprocessor itself, a frequency that is too high will cause the microcomputer/microprocessor to fail (i.e. to exceed the operational frequency design limits).
It is the object of the present invention to provide an oscillator circuit for a microcomputer/microprocessor that provides frequency stability by compensating for variations in transistor characteristics, variations in supply voltage and variations in temperature.